In recent years, image forming apparatus such as copying machines with a multi-stage image magnifying function or a continuous change image magnifying function for forming enlarged or reduced images have been widely used. For example, the U.S. Pat. No. 4,650,317 discloses a copying machine with such a multi-stage image magnifying function and a continuous change image magnifying function.
In the prior art, there are shown a set of magnification up and down keys and a plurality of specific magnification keys. The magnification up and down keys are adapted for continuously increasing and decreasing the current magnification ratio. While the specific magnification keys are adapted for individually selecting a specific magnification ratio from a prescribed number of magnification ratios, e.g., 65%, 78%, 96%, 100% and 122 %. In these image magnifying functions of the prior art, when maximum or minimum magnification ratio is requested, an operator must keep depressing relatively long the magnification up key or the magnification down key, even if he or she has operated the 122 % key or the 65 % key.
Therefore, conventional image forming apparatus such as the copying machine disclosed in the U.S. Pat. No. 4,650,317 were inconvenient for quickly setting the maximum and minimum magnifying ratios, or magnification ratios close to the maximum and the minimum magnification ratios. The specific magnification keys of U.S. Pat. No. 4,650,317 are effective for quickly setting such several intermediate magnification ratios, but they disadvantageously make the construction of a control panel complicated.